Many tennis players try many different techniques to generate spin when they hit the tennis balls. The spin is generated by friction between the string and the ball. Thus, the rougher the string surface is, the easier it is for the player to generate spin. However, most strings, especially the new synthetic strings, are cylindrical in shape and have a smooth surface, making it difficult for players to generate a high speed spin on the tennis balls.
Most strings are made of synthetic fiber and some of them are made of natural gut. Some string manufacturers provide Octagon or Hexagon shaped strings to increase the friction between the string and ball. It may be effective on the main string, but it does not make any difference on the cross string because of the direction of the racquet swing movement. In addition, after some time, the sharp edge of the Octagon and Hexagon will be worn away by wear and tear between the string and ball. Some strings come with a series of small dents on the surface to increase the roughness of the string surface. This is effective when it is new, however, after some time, the small dents will be worn off and the string surface becomes smooth again.
Even though the above mentioned non-cylindrical strings are commercially available, they can be expensive and/or lose some other good quality characteristics compared to traditional cylindrical shaped strings. Cylindrical shaped strings are still the most commonly used strings.